Solid-state lighting solutions based on light-emitting diodes (LEDs) often incorporate phosphors to shift the wavelength of at least a portion of the light emitted by the LED die to another desired wavelength. The wavelength-shifted (or “color-shifted”) light is then emitted from the phosphor alone or color-mixed with another portion of the original light emitted by the LED die. Typical examples include ultraviolet (UV) LEDs with phosphors that emit white light and blue LEDs with phosphors that emit yellow light (which is mixed with blue light to form white light). Thus, phosphor incorporation may be utilized to produce aesthetically pleasing or functionally useful spectral outputs, which may require wavelength(s) that are different from the raw output of the LEDs. Such spectral outputs may be, e.g., warm white, cool white, or light with an increased color rendering index. Phosphors may also be utilized to produce large numbers of spectrally consistent light sources, as the phosphors may be applied to correct for variations in the wavelength of nominally similar LEDs.
Conventional phosphor deposition techniques commonly include direct deposition of the phosphor on LEDs; application of a phosphor slurry on the inside of fluorescent tubes or monochrome cathode ray tubes; photolithographically patterned deposition on the inside of color cathode ray tubes; and encapsulation of the phosphor in polymer films or lenses, which may be positioned adjacent or apart from a light source. Silk-screen-printing techniques may be used to apply thin phosphor dots to planar substrates, without consideration to the form of (or the ability to shape) the edge profiles of the dots. For example, in screen-printing or stencil-printing techniques, the edge profile of the stencil or mask on the screen is generally substantially perpendicular to the surface of the mask or stencil and often formed with a low degree of accuracy and/or reproducibility. Furthermore, the edge profile of the phosphor made by such techniques is generally strongly dependent upon the properties of the phosphor slurry utilized in such methods. These methods typically require a low-viscosity slurry, which tends to spread after removal of the screen print or stencil mask. In some cases the slurry may even spread under the mask edges because of difficulties achieving complete contact between the mask and substrate over an entire large area. Furthermore, the thickness of phosphors deposited by screen-printing techniques is dependent on the mask (or screen) thickness, which is typically not adjustable.
Direct deposition of phosphor slurry over an LED also has disadvantages. Specifically, the final shape and thickness of the phosphor are dependent upon the phosphor slurry viscosity, the elapsed time between deposition and curing, and the curing temperature. Direct deposition generally produces substantially hemispherical phosphor shapes, because these shapes minimize the surface energy of the unconstrained slurry. Thus, direct deposition is generally incapable of achieving an arbitrary phosphor shape. Control of the phosphor shape and thickness are strongly dependent on a range of process parameters, making control and reproducibility difficult to achieve.
Traditionally, LED dies range in size from approximately 300 μm edge-length up to several millimeters in edge length and are packaged individually or in densely packed groups in order to provide lighting packages at the lowest possible cost. In some applications (particularly in arrays), smaller LEDs having dimensions in the range of about 25 μm to about 500 μm (or even smaller) may be utilized. The use of such small LEDs (or arrays thereof) presents challenges for the integration of a phosphor. In order to minimize costs for many applications, the phosphor should be applied near the LEDs rather than over an entire package. Furthermore, the wavelength-conversion efficacy is directly related to its thickness and concentration. In some cases similar results may be achieved by a relatively thicker phosphor layer with a lower phosphor concentration, as with a relatively thinner phosphor layer with a higher phosphor concentration. Moreover, a single uniform shape and size of the phosphor may not adequately address wavelength conversion for multiple LEDs, particularly when one or more of the LEDs emit light from their top and/or side surfaces. For example, a uniformly thick phosphor may result in non-uniform color characteristics as a function of emission angle.
In view of the foregoing, there is a need for techniques for the production of small phosphors with well controlled and repeatable dimensions and geometries suitable for solid-state lighting applications, particularly large arrays of such phosphors for integration with arrays of light-emitting devices.